The present invention relates to a heat exchanger for cooling reaction gas, and includes a tubular connection between a cooled tube and an uncooled tube.
A heat exchanger of this type for cooling reaction gas, and including a tubular connection, is known from U.S. Pat. No. 5,732,981. Reaction gas is produced by a thermal reaction of hydrocarbons in a reaction furnace. Such reaction furnaces are provided with a number of externally heated reaction tubes, through which the hydrocarbons being used are conveyed, accompanied by the addition of water vapor. The reaction gas produced leaves the reaction tubes at a temperature of up to 900° C., and must be cooled very rapidly to stabilize its molecular composition. The rapid cooling of the reaction gas is effected in reaction gas coolers by means of a heat transfer from the reaction gas to evaporating water that is under a high pressure.
With the tubular connection known from U.S. Pat. No. 5,732,981, the end of the uncooled tube is provided with an inlet head, which is widened in a fork-shaped manner and is provided with an inner and an outer tubular section. The intermediate space between the two tubular sections is filled with a heat-insulating material. The outer tubular section is welded to the tubular casing or jacket of the cooled double tube. The inner tubular section is axially spaced from the inner tube of the cooled double tube, whereby a seal, which is embodied as a ring (C, O, U or V shape), is disposed between the end faces of the inner tubular section and the inner tube; the sealing ring is intended to prevent penetration of reaction gas into the heat-insulating material.
A fork-shaped inlet head which is filled with heat-insulating material, for the connection of an uncooled tube with a cooled tube, is also used with the heat exchanger for cooling reaction gas known from EP 810 414 B1. With the known heat exchanger, the cooled tube is comprised of an inner tube that is surrounded in a radially spaced manner by a tubular casing. A water chamber for the supply of a coolant surrounds the inlet end of the cooled tube. The water chamber is comprised of a solid, square or rectangular part into which is introduced a recessed portion having a circular cross-section. The recessed portion accommodates a single cooled tube, whereby the inner tube of the cooled tube is welded into the base of the recessed portion, and the tubular casing is welded to the water chamber. The outer tubular section of the inlet head is welded to the water chamber on that side that faces away from the tubular casing, while the inner tubular section of the inlet head is axially spaced from the inner tube of the cooled tube.
Due to the axial play between the inner tube and the inner tubular section, the known inlet heads permit an unrestricted thermally caused length expansion. The heat insulation that is introduced causes the outer tubular section of the inlet head, which is fixedly connected with the cooled tube, to assume a wall temperature that is below the temperature of the gas flowing through the uncooled tube. The wall temperatures that the tubes reach at the location of connection in this way adapt to one another, thus minimizing the thermal stresses at the location of connection. The sealing ring (C, O, U or V shape) between the inner tubular section and the inner tube prevents penetration of reaction gas into the heat-insulating material of the inlet head. Above 550° C. carbon can precipitate out of the reaction gas and can deposit upon the sealing ring. As a consequence, the sealing ring can leak, so that reaction gas can penetrate into the heat-insulating material. As a further consequence, carbon precipitating out of the leakage stream of the reaction gas can deposit upon the heat-insulating material, leading to buckling stresses in the inner tubular section, and to circumferential stresses in the outer tubular section of the inlet head.
The object of the present invention is to configure the heat exchanger of the aforementioned type, which includes a tubular connection between an uncooled and a cooled tube, in such a way that the sealing relative to the reaction gas is improved when carbon is precipitated.